1. Field of the Invention
The present invention relates to a plasma processing apparatus, such as an ion source, a CVD apparatus, an etching apparatus, an X-ray source, or the like.
2. Description of the Related Art
In a plasma processing apparatus, a gas is electrically excited to generate plasma, and an object under treatment is processed by utilization of the plasma. In this type of apparatus, a voltage is applied between electrodes made of conductive metals and a gas is introduced into the region between the electrodes, for exciting.
For example, in the electron beam excitation ion source of an ion implantation apparatus which implants ions into a semiconductor wafer as impurities, plasma is produced from a predetermined gas atmosphere by causing discharge between the filament and electrode. A predetermined raw gas (e.g., a BF.sub.3 gas) is introduced into an ion generation chamber formed by a high-melting point conductive metal (e.g., molybdenum). Electrons are extracted from the plasma by means of an ion extraction electrode, and are supplied into the raw gas, so as to obtain ions from the raw gas. The ions, thus obtained, are implanted into the semiconductor wafer.
In the ion source which was mentioned above as one type of plasma processing apparatus, the walls of the ion generation chamber are liable to damage due to the action of sputtering or etching caused by the plasma, and particles separating from the walls of the ion generation chamber are liable to attach to undesirable portions of the ion source, such as the ion extraction electrode.
This problem is particularly noticeable when the raw gas used in the plasma processing is highly corrosive, just like BF.sub.3. Moreover, in the case where the walls of the ion generation chamber are made of molybdenum and the raw gas is BF.sub.3, an insulating material, such as molybdenum fluoride, attaches to the ion extraction electrode, forming an insulating film on the surface of the ion extraction electrode. In order to remove such an insulating film from the ion extraction electrode, the apparatus has to be maintained frequently, resulting in a low rate of operation of the apparatus and poor productivity.